Field of the Invention
The invention relates in general to a portable electronic device, and more particularly, to a touch sensing circuit of a portable electronic device.
Description of the Related Art
Operating interfaces of recent electronic products have become increasingly user-friendly and intuitive with the progressing technology. For example, through a touch screen, a user can directly interact with applications and input messages/texts/patterns with fingers or a stylus, thus eliminating complexities associated with other input devices such as a keyboard or buttons. A touch screen usually comprises a transparent sensing panel and a display panel disposed at the back of the sensing panel. According to a user touch position on the sensing panel and a currently displayed image on the display panel, an electronic apparatus determines an intention of the touch to execute corresponding operations.
Conventional touch sensing technologies can be roughly categorized into resistive, capacitive, electromagnetic sensing, ultrasonic and optical types. Among the above, capacitive touch technologies are most commonly applied in portable electronic devices. In self-capacitive touch technologies, the position of a touch point is determined according to a capacitance change in a sensing electrode caused by a user touch. In general, each sensing electrode has a background capacitance value when not touched by a user. The capacitance change refers to a difference between the capacitance value caused by the user and the background capacitance value. Ideally, the background capacitance value should remain substantially unchanged. However, in real situations, at the instant of a significant change in the ambient temperature, the background capacitance value frequently significantly changes. Before the background capacitance value is corrected, a backend circuit may mistake a point that is not touch as a touch point or a user touch point as a point that is not touched, hence triggering an unintended operation result.
FIG. 1 shows an example of a background capacitance change. Before the time point t1, the background capacitance value of a sensing electrode is “a”. Assuming that, when a capacitance value of a sensing electrode is detected as higher than the background capacitance value and the difference between the two is higher than a predetermined value Δ, a backend circuit is configured to determine that the sensing electrode receives a user touch. In the above situation, if the capacitance value of one sensing electrode is higher than the capacitance threshold b shown in FIG. 1, the backend circuit determines that the sensing electrode receives a user touch. After the time point t1, due to a drastic change in the ambient temperature of the electronic device, the background capacitance value of the sensing electrode also starts rising, becomes stable at the time point t2 and is then kept at “c”. As seen from FIG. 1, even when the sensing electrode is not touched by the user, the backend circuit may still misjudge that the sensing electrode receives a continual user touch as the capacitance value of the sensing electrode is higher than the capacitance threshold b.